1. Field of the Invention
This invention is concerned with ground-contacting drive means of different types as used with off-road vehicles in seismic exploration.
2. Background of the Invention
A large amount of seismic exploration is conducted off-road in hostile terrain. For such operations, specialized vehicles have been developed for heavy duty service in connection with recording-instrument units, bore-hole drilling rigs and seismic vibrators.
Depending upon the service, some vehicles are equipped with oversized wheels using tire sizes such as 66.times.44 or larger. Such large tires provide minimal ground loading, good traction over ice, a certain amount of floatation in swampy regions, yet provide acceptable road speed when moving from one operational base to another. Usually each pair of wheels, front and rear, are driven independently by individual hydrostatic drive motors. Generally the vehicles are articulated. That is, the vehicle consists of a front and a rear bed frame that are joined by a suitable swivel joint. The unit is steered by hydraulic rams that bend the bed frames relative to each other about a vertical axis. In that arrangement, the wheels and wheel transaxles are not themselves steerable but are fixedly secured to their respective bed frames. See for example, U.S. Pat. No. 3,905,446.
Wheels, even oversized, are not always suitable for some operational areas. In deep snow in the northern reaches of Alaska and Canada, and in some very sandy desert areas, track drive means are preferred. Although track drives are much heavier than wheels, the large track area substantially reduces the vehicle loading per unit area.
Known tracked vehicles employed in seismic exploration are designed solely for use with tracks. See for example, U.S. Pat. No. 3,717,217. Many of the heavier units employ an all-track configuration, much like conventional crawler-type tractors. Steering is accomplished by differential braking of one track with respect to the other. That type of steering is known to the trade as "skid steering". Skid steering has disadvantages. In climbing a slope where a turn is required, the stalled track looses traction such that oft-times the vehicle cannot negotiate the slope. Further, when the vehicle pivots about the braked track, the ground beneath, is severely disturbed, something that must be avoided in fragile environments.
Other types of units may be articulated as with the earlier mentioned wheeled vehicles, with each bed frame having its own set of tracks. The two sets of tracks are driven by a central engine through drive-shafts having U-joint coupling at the point of articulation.
An articulated track unit is generally about six to eight tons heavier than an equivalent wheeled vehicle. Such units are very slow when driven over haul-roads from one site of operations to another. However, they do not tend to stall on slopes and they are kind to fragile terrain.
In one area of operations along the North Slope of Alaska, seismic lines of survey extend from land, offshore into the Beaufort Sea. On land, tracked vehicles are used. When venturing onto the sea ice in winter, wheeled vehicles are preferred over tracked vehicles because they are lighter in overall weight and can operate faster. For such Arctic operations, two sets of certain types of the vehicles such as vibrators must be maintained, one set having wheels, the other set having tracks. That need results in substantially increased capital costs. Therefore, it would be most useful to provide a heavy-duty off-road vehicle that can be quickly and easily converted in the field from wheels to tracks and vice versa.
Conversion kits are known, of course, particularly for use with tandem rear drive-wheels of a conventional truck, where an endless track is wrapped around each tandem pair of the rear wheels. The truck is in effect converted to a half-track of military fame. The truck is then steered by conventional means, the front wheels either being left intact or equipped with skis. Similar conversion kits are available for use in converting motor bikes into snow scooters. For that conversion, the rear wheel of the bike is removed and is replaced by a track assembly. Typical of such patents are U.S. Pat. Nos. 1,453,782; 3,381,403 and 3,439,763.
Considering the physical size of the 66.times.44 tires used on seismic vehicles, a track assembly large enough to enwrap both pairs of drive wheels would be too cumbersome to be practical.
Another type of drive-means conversion which, although not necessarily suitable to this application but which is mentioned for completeness, is the conversion of road vehicles for use on rails. Small rail wheels bolted under the chassis of a road vehicle, are lowered to engage the pair of rails. The rear tires of the road vehicle then drive the rear rail wheels by frictional contact therewith to propel the vehicle as desired.